A hot melt adhesive is a solvent-free adhesive and has a characteristic that instant bonding and high-speed bonding can be achieved since adhesiveness is exhibited after the adhesive is melted by heating and coated onto an adherend, followed by cooling to solidify the adhesive, and therefore has been used in a wide range of fields. Not only adherends to be bonded with such a hot melt adhesive but also the conditions of use thereof are various. At present, various hot melt adhesives for use in various applications have been developed and supplied to the market. Also for the conditions of use, various operating temperatures ranging from low temperatures to high temperatures are considered, and recently, a hot melt adhesive having excellent heat resistance has been demanded. Further, in consideration of the wettability to a base material or the coatability onto a base material having a complicated shape, the hot melt adhesive is required to have an appropriate length of open time. Here, the “open time” refers to a tackiness retention time from when an adhesive is coated onto an adherend to when the temperature is decreased and the tackiness disappears, and from the viewpoint of the workability, it is preferred that the open time is longer. If the open time is too short, it is difficult to bond an adherend to a base material.
Thus, the development of a hot melt adhesive which is well balanced between heat resistance and a moderate open time and has a favorable coatability has been desired.
Conventionally, there has been known a hot melt adhesive obtained by blending a tackifier resin, a liquid plasticizer, and other additives in a copolymer rubber or the like (see, for example, PTL 1). However, a conventional hot melt adhesive has a problem that when the amount of a plasticizer, other additives or the like is increased to prolong the open time, the cohesive force is decreased, while on the other hand, when the amount of a tackifier resin or the like is increased to improve the cohesive force or the ability to maintain heat resistance, the flexibility is decreased.
Further, PTL 2 discloses a hot melt adhesive containing an ethylene-based copolymer as a base polymer and also containing a tackifier resin and a wax. This adhesive has high adhesive strength at low temperatures, but its open time is insufficient and its heat resistance, particularly heat creep resistance is poor as an adhesive.
PTL 3 discloses that in order to improve the heat resistance of a hot melt adhesive, a specific polymer (a functionalized metallocene polymer) is used as a base polymer. However, the hot melt adhesive described in PTL 3 is insufficient in the open time and does not fully satisfy the heat resistance requested by users.